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Water as a
Resource
Chapter 11
Lake Mead, immediately behind Hoover (or Boulder) Dam, an artificial
reservoir created on the Colorado River, border between Nevada and
Arizona. Photo is from Colorado River Commission of Nevada.
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The Global Water Budget
Consider water as a resource because it isimportant for domestic use, agriculture,
and industry
Fresh water is limited on the earth Mostly Polar ice and in the ground
Water is regionally a renewable resource
Locally water may not be renewable
The geologic condition affects the quality
and quality of water in a region
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Fluid Storage and Mobility
Porosity andpermeability involve theability of rocks/sediments/soils to contain
fluid and to allow fluids to pass through
them
Porositythe proportion of void space
(holes and/or cracks) in material (soil or
rock) where fluid can be stored
Usually expressed as a percent (1.5%) or a
decimal (0.015) of the entire volume
Pore space can be occupied by fluid or gas
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Fluid Storage and Mobility
Permeabilitymeasures how readily afluid passes through a material
Measures degree of interconnection between
pores and cracks in rocks and soil
Grain shape and size are factors
How grains fit together influences permeability
also
Porosity andpermeability play a big role
in groundwater hydrology, oil and gas
exploration, and nuclear waste disposal
R d d ti l f l i id i
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Rounded particles of equal size provide maximum
porosity
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Fluid Storage and Mobility
Rock type will impact porosity and
permeability
Igneous, metamorphic, and chemical sedimentary
rocks have crystals that are tightly interlocked and
low permeability and porosity
Weathering, dissolution, and fracturing will
increase porosity and permeability in crystalline
rock and carbonate rocks
Clastic sediments have more porosity and
permeability
Sandstones are generally very porous
Clay and mud rich rocks are not porous or
permeable
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Subsurface Waters Soils which are permeable will allow excess precipitation
to infiltrate Gravity will draw water down until an impermeable layer, an
aquitard, halts it
Above this layer ground water will accumulateinfilling porespaces
The saturated zone (or phreatic zone)will fill with water Ground water is stored
The unsaturated zone (or vadose zone)lies above thesaturated zone and pore spaces are filled with water Soil moistureis found
The water tableseparates the two zones; it is the top of thesaturated zone
Water stored and transmitted at rates sufficient enough tobe useful is called an aquifer Water moving into an aquifer to is called recharge
Th di ti ti b t th t t d d
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The distinction between the unsaturated and
saturated zones
A ffl t t i t f th t
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An effluent stream receives water from the water
table
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Aquifer Geometry and
Groundwater Flow
Geology and geometry of rocks and sediments
will control the behavior of ground water
An aquifer without an aquitard above it is an
unconfined aquifer An aquifer with an aquitard above and below is a
confined aquifer
A confined aquifer may see hydrostatic water
pressure increase and form an artesian system Drilling into a confined aquifer under pressure will see the
water rise above the aquifer
In this system the potentiometric surfaceis the height to
which the water would rise
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An artesian well taps into groundwater under
pressure, in this case due to elevation head and
the bounding of the aquifer by aquitards
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Darcys Law
Darcys Law is Q = KA(h/l)
Q = discharge
K = hydraulic conductivity
A = cross-sectional area Dh = difference in hydraulic head
Dl = distance between well heads
Discharge is the amount of water flowing past a
point over a period of time
It is influenced by the porosity and permeability of the
rock or sediment of the aquifer
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Potentiometric surface
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Groundwater Flow Hydraulic headis potential energy in an
aquifer
The height of water in an unconfined aquifer
reflects the hydraulic head
The higher the water table the higher the head
The higher the potentiometric surface in a
confined aquifer will equate to higher hydraulic
head
Ground water flows spontaneously from areas
of high hydraulic head to areas with low
hydraulic head
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Other Factors in Water Availability
Geometry of host rock units
Distribution of aquitard lenses may form
perched water tab les
Local precipitation patterns and
fluctuations
Minerals in host rock Location of wells relative to recharge
zones and discharge points
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Consequences of Groundwater
Withdrawal Pumping ground water will lower the water table and
form a cone of depressionaround the well This may impact local and regional ground water availability
Aquifer rocks may re-adjust after ground water is
withdrawn Sediments may compaction and cause surface subsidence
An area of low elevation, relative to sea level, may be inundatedby the sea
Sinkholes may also develop depending on the host rock
Near coastlines saltwater intrusion may occur Freshwater is less dense than saltwater
Saltwater near a coast line may push freshwater lenses back ifrecharge is not sufficient to force seawater toward the sea
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Figure 10.11b
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Other Impacts of Urbanization on
Groundwater Systems An increase in people in an area may cause
ground water supplies to be exhausted or lossof rechargeto occur Pavement and parking lots reduce the effectiveness
of water infiltrating into ground water Building on wetlands reduces recharge, water
storage, and water quality
Ground water recharge can be enhanced byincorporating various artificial recharge
strategies Build artificial recharge basins
Employ any method to slow down run off andincreases surface water infiltration
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Other Features Involving
Subsurface Water Ground water may dissolve large volumes of
rock (soluble rock) Collapse of the surface rock may result in sinkholes
Caverns may also be enlarged
Karstis a type of land form associated withmany sink holes in soluble bedrock such aslimestone, dolomite, or gypsum Water removes the minerals of the rock and carries
the ions off in solution
Ground water flow rates will increase in karst areas Ground water flows faster without sediments and rock in the
flow path
Pollutants move faster through ground water systems in karstareas
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Water Quality
Measures for expressing Water Quality
Parts per million (ppm)
Parts per billion (ppb)
Total Dissolved Solids (TDS) The sum of dissolved solid chemicals in the water
It is important to know what chemicals aredissolved!
Hard Watercontains substantial amounts ofcalcium and magnesium
Greater than 80 to 100 ppm
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Water Use and Water Supply
In U.S. the east is generally humid
More dependence is on surface water
In the west more arid condition are found
More dependence is on ground water andimpounding surface water for storage
Global water usage
Too many people Too much demand
Not many places to find more water
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Case Studies in Water Consumption Plenty of water-supply problems: lakes, streams, or
ground water
The Colorado River Basin Drains portions of seven western states and many of these states
have extremely dry climates
Is an international stream and Mexico is entitled to some of itswater
The High Plains (Ogallala) Aquifer System The Ogallala Formation, a sedimentary aquifer, underlies most ofNebraska and sizeable portions of Colorado, Kansas, and theTexas and Oklahoma panhandles
The most productive units of the aquifers are sandstones andgravels
The Aral Sea, a disappearing lake Lies on the border of Kazakhstan and Uzbekistan
Lake Chad A disappearing lake on the edge of the Sahara Desert
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Extending the Water Supply
Conservationa must do strategy in U.S. Water is wasted every day in different ways
Interbasin Water Transfer
Conservation alone will not resolve the
imbalance between demand and supply
Moving surface waters from one streamsystems drainage basin to anothers
where demand is higher Desalination
Improve and purify waters not now used andmake them usable
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